The present invention relates to glass and ceramic vessels and, more explicitly to glass and ceramic vessels which are coated with an organic polymer to improve the mechanical strength (resistance to breakage upon impact) of the vessel and to minimize scattering of fragments should the vessel be fractured.
Glass vessels, such as glass bottles and laboratory glassware, have long been used as containers for various foodstuffs and as receptacles for use in laboratory apparatus inasmuch as they are superior in manY respects to vessels fabricated from other materials. Two recognized drawbacks to the glass vessel, however, are its susceptibility to fracture upon impact and its tendency to shatter upon fracture to produce sharp glass shards. The inherent susceptibility of glass to fracture is heightened by the development of scratches and pits in the surface thereof resulting from mechanical abrasions and/or chemical attack encountered during use. The safety hazard involved in the breakages of glass vessels is magnified in those applications where the contents of the vessel are under pressure, e.g., bottles containing carbonated beverages, inasmuch as the shards resulting from such breakage will be scattered with great force over a relatively wide area.
It was found that breakage of glass vessels and the scattering of glass shards therefrom could be significantly reduced by applying a coating of an organic polymer onto the vessels. Furthermore, the coating protected the surface of the vessel from surface abrasions and chemical attack; hence, substantially extending the useful life thereof.
Various methods have been devised for applying such coatings. For example, a coating or film of a liquid organic polymeric material can be applied through such conventional processes as dipping, painting, or spraying; which coating or film is cured through such well known practices as simple air drying, defined heat treatments, and exposures to ultraviolet or other actinic radiation. Sheaths or sleeves of heat-shrinkable thermoplastic materials have been applied to bottles and the so-coated bottles then heated to shrink the sheaths or sleeves around the bottles in a snug-fitting relationship. As can be appreciated, combinations of those two general methods have been utilized.
Devising protective coatings for laboratory glassware has been particularly critical inasmuch as the breakage of glass vessels containing corrosive or toxic liquids is a major safety problem. One process which is widely practiced commercially contemplates coating the vessel by heating to a temperature of about 300.degree. C., dipping into a fluidized bed filled with powder of a suitable thermoplastic polymer, e.g., plasticized polyvinyl chloride (PVC) or ethylene-vinyl acetate copolymer, and thereafter withdrawing from the fluidized bed. The powder sticks to the hot glass surface and then flows to form a clear, coherent, continuous coating. In general, coatings have been prepared by that process having thicknesses of about 0.008"-0.105" (0.2-2.7 mm).
Such coatings can be applied either to an untreated glass surface, in which case the bond between the glass and the coating is relatively weak, or it can be applied to a glass surface which has been treated with a chemical primer which reacts stronglY (during the powder fusion step) with both the glass and the applied polymer powder coating.
Strong adhesion of the thermoplastic coating to the glass surface imparts at least the four following advantages when compared to coatings where the bond between the polymer and the glass is weak:
First, it improves the durability of the coating, i.e., it reduces the tendency of the coating to part from the glass surface, when the vessel is subjected to mechanical abuse or to such activities as dishwashing and autoclaving;
Second, a strongly adhered coating improves the overall mechanical strength of the glass vessel making it more resistant to breakage by impact;
Third, a strongly adhered coating is much less likely to trap liquids in the interfacial area between the glass and the polymer skin; and
Fourth, a strongly adhered coating is much less likelY to be removed inadvertently by users who "pick" at it while using the coated vessel.
Nevertheless, strong adherence of the coating does impart one recognized serious disadvantage; viz., it reduces the ability of the polYmer skin to contain the broken glass shards and liquid contents when the vessel is fractured. Thus, experience has shown that this reduction in containment ability is due to the fact that a strongly adhered coating is much more likely to be cut by the attached glass shards. Stated in another way, because the polymer coating is strongly bonded to the glass surface, the shards resulting from the impact fracture penetrate through the coating, thereby dramatically reducing the tear strength of the polymer.
Accordingly, the primary objective of the present invention was to devise polymer coatings for glass and ceramic vessels which would preferably be transparent, which would preferably resist the development of haze and/or discoloration; which would resist delamination, i.e., separation from the vessel surface when subjected to washing and exposure to an autoclave atmosphere; which would prevent the trapping of liquids in the interfacial area between the vessel surface and the polymer skin; and which, upon fracture of the glass or ceramic, would contain the resultant fragments and any liquid contents therein. Stated more simply, the primary objective of the present invention was to develop polymer coatings for glass and ceramic vessels which would demonstrate the several advantages of strongly adhered coatings described above, while concurrently exhibiting dramatic improvement in retaining broken fragments, and, hence, in retaining liquids within glass vessels upon breakage thereof.